Can we assume you are keeping records of all your sales?
You need to keep the selling prices in both currencies-- in two different columns.
You can set up your Paypal account to keep your payments received in US dollars separate from those received in loonies.
If your buyer buys an item from your dotCOM, it will go to your Paypal US dollar balance.
If your buyer buys an item from your dotCA, it will then go automatically to your Paypal Canadian balance.
Unless you have an account in a US bank, it is expensive to move US funds from Paypal to your Canadian US dollar account.
Allow them to build up and use them for buying, and only move them to your Canadian account when the exchange rate is favourable.
If you try to move U$ from Paypal to your Canadian account,it will be exchanged for Canadian dollars (with a fee) then once in Canada those loonies will be exchanged back into U$ and deposited in your Canadian US dollar account.